The present invention relates to instruments for use with lead acid storage batteries and more particularly, to a method and apparatus for determining the capacity of a lead acid storage battery.
Lead acid storage batteries are used as motive power on fork lift trucks, tow tractors, mining vehicles, golf carts and other electric vehicles. In many of these uses, particularly industrial materials handling, the batteries are not continuously charged as they are, for example, in automobiles. Instead, they are normally discharged in operation and then recharged and readied for another discharge cycle.
The need for some form of battery protection against damage from overcharging and progressive undercharging has been recognized for many years. Existing devices for this purpose have been available under such names as discharge warning indicator, battery condition indicator, battery energy saver, battery discharge control, expanded scale voltmeter, battery protector, and battery fuel gauge. Such devices are generally used in combination with a hydrometer reading of the specific gravity of the battery electrolyte. Generally, this measurement is taken just prior to changing on a single "pilot" cell selected on a rotational basis. This measurement is used to monitor the accuracy of whatever other battery protection device is installed.
Typically, such devices have, in practice, proven inadequate or impractical for one or more of the following reasons:
1. If fitted with a gauge or meter, the device may be over-sensitive to the application or workload. Heavy lifting, higher stacking, frequent lifting, and driving rapidly up a ramp with a load, can cause a sudden voltage drop that makes the meter indicator needle swing erratically.
2. If given only a simple warning light, the operator has no way of judging how close he is getting to the warning point. Such lights usually indicate the beginning of a preset period of time, normally two to fifteen minutes, before a relay acts to disable lifting and possibly other truck functions. This time may be insufficient for the operator to reach a convenient point in his work before charging. In addition, it is not uncommon for the operator to react to the building time pressure with aggression by damaging or sabotaging the device.
3. Generally such devices measure or monitor cell voltage under load; I.E., during truck operation. As indicated above, momentary surges in this voltage due to lifting, etc. tend to trigger the lift lockout switch as the battery approaches 50% discharge. To avoid this, in recent years such devices have commonly averaged the voltage under load over some period of time such as, three minutes. However, the devices may still trigger the lockout switch prematurely because the loaded voltage is affected by the discharge rate. In other words, a given battery may exhibit a given final voltage over a broad range of levels of charge, i.e., from 90% charge to 20% charge. Lift lockout, therefore, remains premature or unreliable and the battery may still be damaged by excessive discharge.
4. Some devices can be manually reset or overridden by the operator. Sometimes a manual "reset" button is provided for the purpose, or the operator may find he can "fool" the device into sensing it has been connected to a recharged battery by simply unplugging and replugging from the battery which needs recharging. Other devices provide for adjustment of the warning and/or reset point which can be counter-productive. Finally, operators may simply short out the device in a way that makes it difficult to determine whether vandalism has occurred.
5. Devices that require or permit calibration by the user may not be properly set for the battery type and workload conditions under which they are employed.
6. Some devices may either not sense or not indicate the temporary increase in available capacity due to "recovery" or rest. If such increases in capacity are not recognized, they tend to go unutilized.
7. Many battery protection devices restrict operations by requiring a battery that is 80-100% recharged before they will reset after lift lockout. While it is generally not considered good practice to send a truck out with a partially charged battery, this could be desirable near the end of the final shift of the day.
8. Finally, battery protection devices that are properly adjusted for new batteries may fail to operate as desired when connected to older or less well-maintained batteries. Such batteries typically exhibit a lower specific gravity when fully charged and reach the specific gravity equivalent to their recharge point in less time.